This invention relates generally to electronic photodiodes, and more particularly relates to avalanche photodiodes such as Geiger-mode avalanche photodiodes.
Avalanche photodiodes (APDs) operating in Geiger-mode (GM) can be employed to detect single infrared photon arrival with sub-nanosecond accuracy. As a result, Geiger-mode avalanche photodiode arrays are receiving increased interest for a number of photon-counting applications, including astronomy, three-dimensional laser radar (LADAR), and photon-counting based optical communication.
In Geiger-mode operation, an avalanche photodiode is biased above its characteristic breakdown voltage. This is a metastable state because the generation of an electron-hole charge pair in the photodiode, either thermally or through absorption of a photon, can cause the photodiode to break down. For example, upon absorption of a photon at a thusly biased photodiode, breakdown produces a rapid rise in current, which ultimately becomes limited by series resistance and internal space-charge effects. Because an avalanche photodiode when operated in Geiger mode is initially biased a few volts above breakdown, the breakdown event caused by photon absorption produces a large voltage signal swing that is sufficient for directly driving CMOS digital logic.
This is an important attribute of Geiger-mode APDs and has allowed the development of Geiger-mode arrays bonded directly to readout integrated circuits (ROICs) and micro-optics to form focal plane arrays for use in imaging or other applications. The ability to produce arrays of photodiodes and read them out at high data rates is important for both LADAR and optical-communications applications. The use of an all-digital readout reduces power, and makes the APD technology more easily scalable to large array sizes than competing technologies employing, e.g., linear-mode APDs or photomultiplier tubes.
One limitation of such densely packed Geiger-mode APDs arrays is optical cross-talk. When operated in or near Geiger-mode, avalanche photodiodes generate many highly energetic electron/hole charge carrier pairs. Some of these charge carriers lose energy by emitting within the photodiode itself a spectrum of photons, which can be detected at other nearby photodiodes in an array of photodiodes. Such detection of photons that are secondary, i.e., produced at and coming from a neighboring photodiode rather than from a source external to the photodiode array, cause corresponding secondary detection events across the photodiode array. Cross-talk is the term used herein to describe this process of secondary photon detection across an APD array. As Geiger-mode APD array size, density, and performance requirements increase, optical cross-talk becomes an increasingly limiting source of such secondary photon detection.